King was a champion of non-violent protest for social change, and his writings and speeches still resonate for many activists.

"Because of the leadership of this man we rose up out of fear and became willing to put our bodies on the line," said John Lewis, a Democratic congressman and a companion of King's in the civil rights struggles of the 1960s.

Democratic presidential candidate Hillary Clinton and Republican hopeful John McCain both visited Memphis for the commemorations, including a "recommitment march" and the laying of wreaths at the site of King's assassination.

McCain visited the motel where King died, while Clinton said King had "ledme to confront a world bigger and broader than the one I inhabited".

"It wasn't a revolution of guns. It was a revolution of hearts and minds of attitudes and action."

Clinton's Democratic rival and current frontrunner, Barack Obama, was not present in Memphis as he was campaigning in Indiana, however he gave speech saying King "through his faith, courage, and wisdom ... moved an entire nation.

"He preached the gospel of brotherhood; of equality and justice. That's the cause for which he lived - and for which he died 40 years ago," he said.

History of struggle

The National Civil Rights Museum opened in 1991 at the former motel, which now holds most of the exhibits tracing the history of the struggle for equal rights.

The museum also encompasses the hotel from which confessed killer, James Earl Ray admitted firing the fatal shot. Ray, who later recanted his confession and protested his innocence, died in prison in 1998.

In Indianapolis, Ethel Kennedy, the wife of Robert Kennedy - the former presidential candidate who was also assassinated in 1968 - was scheduled to speak at what is now Martin Luther King Jr Park.

Her husband gave a passionate speech there the night of King's assassination that was credited with quelling violence in the city that erupted in response to King's killing.